This invention relates to aluminum base alloys, and more particularly, it relates to improved lithium containing aluminum base alloys, products made therefrom and methods of producing the same.
In the aircraft industry, it has been generally recognized that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, lithium additions have been made. However, the addition of lithium to aluminum alloys is not without problems. For example, the addition of lithium to aluminum alloys often results in a decrease in ductility and fracture toughness. Where the use is in aircraft parts, it is imperative that the lithium containing alloy have both improved fracture toughness and strength properties.
It will be appreciated that both high strength and high fracture toughness appear to be quite difficult to obtain when viewed in light of conventional alloys such as AA (Aluminum Association) 2024-T3X and 7050-TX normally used in aircraft applications. For example, a paper by J. T. Staley entitled "Microstructure and Toughness of High-Strength Aluminum Alloys", Properties Related to Fracture Toughness, ASTM STP605, American Society for Testing and Materials, 1976, pp. 71-103, shows generally that for AA2024 sheet, toughness decreases as strength increases. Also, in the same paper, it will be observed that the same is true of AA7050 plate. More desirable alloys would permit increased strength with only minimal or no decrease in toughness or would permit processing steps wherein the toughness was controlled as the strength was increased in order to provide a more desirable combination of strength and toughness. Additionally, in more desirable alloys, the combination of strength and toughness would be attainable in an aluminum-lithium alloy having density reductions in the order of 5 to 15%. Such alloys would find widespread use in the aerospace industry where low weight and high strength and toughness translate to high fuel savings. Thus, it will be appreciated that obtaining qualities such as high strength at little or no sacrifice in toughness, or where toughness can be controlled as the strength is increased would result in a remarkably unique aluminum-lithium alloy product.
U.S. Pat. No. 4,626,409 discloses aluminum base alloy consisting of, by wt. %, 2.3 to 2.9 Li, 0.5 to 1.0 Mg, 1.6 to 2.4 Cu, 0.05 to 0.25 Zr, 0 to 0.5 Ti, 0 to 0.5 Mn, 0 to 0.5 Ni, 0 to 0.5 Cr and 0 to 2.0 Zn and a method of producing sheet or strip therefrom. In addition, U.S. Pat. No. 4,582,254 discloses a method of superplastically deforming an aluminum alloy having a composition similar to that of U.S. Pat. No. 4,626,409. European Patent Application 210112 discloses an aluminum alloy product containing 1 to 3.5 wt. % Li, up to 4 wt. % Cu, up to 5 wt. % Mg, up to 3 wt. % Zn and Mn, Cr and/or Zr additions. The alloy product is recrystallized and has a grain size less than 300 micrometers. U.S. Pat. No. 4,648,913 discloses aluminum base alloy wrought product having improved strength and fracture toughness combinations when stretched, for example, an amount greater than 3%.
The present invention provides an improved lithium containing aluminum base alloys which permit products having improved strength characteristics while retaining high toughness properties.